1. Field of the Invention
The present invention relates to a substrate which is used in a semiconductor element. The present invention also relates to a process for producing a substrate which is used in a semiconductor element. The present invention further relates to a semiconductor element which uses the above substrate.
2. Description of the Related Art
Japanese Journal of Applied Physics Vol. 37 (1998) Part 2, pp. L1020 discloses a short-wavelength semiconductor laser device which emits laser light in the 410 nm band. This semiconductor laser device is produced as follows. First, a GaN layer is formed on a sapphire substrate, a stripe pattern of a SiO2 film is formed on the GaN layer, and a GaN thick film is formed by selective lateral growth. In the selective lateral growth, stripe areas of the GaN layer which are not covered by the stripe pattern of the SiO2 mask are used as nuclei of the growth. Then, a GaN substrate, which is called ELOG (epitaxial lateral overgrowth) substrate, is obtained by separating the sapphire substrate from the GaN thick film. Next, an n-type GaN buffer layer, an n-type InGaN crack prevention layer, an n-type AlGaN/GaN modulation doped superlattice cladding layer, an n-type GaN optical waveguide layer, an n-InGaN/InGaN multiple-quantum-well active layer, a p-type AlGaN carrier block layer, a p-type GaN optical waveguide layer, a p-type AlGaN/GaN modulation doped superlattice cladding layer, and a p-type GaN contact layer are formed on the GaN substrate.
In order to obtain a semiconductor laser device which is reliable in high output power operation, a portion of the substrate on which an optical waveguide is formed is required to be a low-defect region. In addition, in order to obtain a high output power semiconductor laser device, the semiconductor laser device is required to have a wide stripe structure. Therefore, in order to achieve high reliability in a high output power semiconductor laser device having a wide stripe structure, it is necessary to form the semiconductor laser device by using a GaN substrate including a wide low-defect region.
However, since the ELOG disclosed in Japanese Journal of Applied Physics Vol. 37 (1998) Part 2, pp. L1020 is formed by the selective lateral growth using as nuclei the stripe areas of the GaN layer which are not covered by the stripe pattern of the SiO2 film, defects are suppressed in regions formed by growth on the GaN layer. However, since the density of the nuclei for growth is high, the spaces between the nuclei are bridged before the grown nuclei become large, and therefore the defect density in the bridged regions becomes high. That is, it is difficult to form a wide low-defect region. In addition, when the thickness of the GaN thick film is increased, the defect density is further increased, and therefore it is further difficult to form a wide low-defect region. Thus, for example, in oscillation in the fundamental transverse mode, the highest output power obtained by reliable semiconductor laser devices using the ELOG substrates is about 30 mW.
Further, generally, reliability of a semiconductor element in which semiconductor layers are formed on a substrate depends on the defect density in the substrate. Therefore, a substrate including a wide low-defect region is required for every type of semiconductor element.
An object of the present invention is to provide a substrate which is used in a semiconductor element, and in which the defect density is low in a wide region.
Another object of the present invention is to provide process for producing a substrate which is used in a semiconductor element, and in which the defect density is low in a wide region.
A further object of the present invention is to provide a semiconductor element which uses a GaN substrate in which the defect density is low in a wide region.
(1) According to the first aspect of the present invention, there is provided a process for producing a substrate for use in a semiconductor element, comprising the steps of: (a) forming on a surface of a base substrate a porous anodic alumina film having a plurality of minute pores; (b) etching the surface of the base substrate by using the porous anodic alumina film as a mask so as to form a plurality of pits on the surface of the base substrate; (c) removing the porous anodic alumina film; and (d) forming a GaN layer on the surface of the base substrate by crystal growth.
According to the second aspect of the present invention, there is provided a process for producing a substrate for use in a semiconductor element, comprising the steps of: (a) forming on a surface of a base substrate a porous anodic alumina film having a plurality of minute pores; (b) etching the surface of the base substrate by using the porous anodic alumina film as a mask so that a plurality of pits are formed on the surface of the base substrate and the porous anodic alumina film is etched off; and (c) forming a GaN layer on the surface of the base substrate by crystal growth.
(2) The processes according to the first and second aspects of the present invention may have the following additional features.
(i) In each of the processes according to the first and second aspects of the present invention, it is preferable that the GaN layer is formed on the surface of the base substrate so that the plurality of pits are partially filled with the GaN layer and a plurality of spaces are left in the plurality of pits.
Alternatively, the GaN layer may be formed on the surface of the base substrate so that the plurality of pits are fully filled with the GaN layer i.e., no space is left in the plurality of pits.
(ii) The GaN layer can be formed by crystal growth so as to partially fill the plurality of pits when the depths of the plurality of pits are appropriately determined in consideration of the diameters and the total area of the plurality of pits. Specifically, the depths of the plurality of pits are preferably 300 nm or greater, and more preferably 500 nm or greater.
(iii) As disclosed in Japanese Journal of Applied Physics Vol. 35 (1996) Part 2, pp. L126, the porous anodic alumina film is an alumina film having a great number of minute pores which is obtained by selectively etching off bottom portions of an alumina film by anodic oxidation so that the great number of minute pores (perforation holes) are formed. The porous anodic alumina film having a great number of minute pores can be obtained by forming an aluminum thin film on a base substrate, and anodizing the aluminum thin film. Alternatively, the porous anodic alumina film having a great number of minute pores can be separately produced, and placed on the base substrate.
(iv) For the purpose of decreasing the defect density by control of the density of the nuclei for growth, and suppressing the occurrence of defects in the bridge regions between the nuclei, the diameters of the minute pores arranged on the surface of the base substrate are preferably 10 to 400 nm. In addition, for a similar purpose, it is preferable that the total area of the minute pores occupies 50 to 90% of the entire surface of the base substrate.
(v) The process according to each of the first and second aspects of the present invention may further comprise an additional step of forming as an uppermost layer a conductive GaN layer which is doped with a conductive impurity.
(vi) The process according to each of the first and second aspects of the present invention may further comprise an additional step of removing the base substrate. Further, all of the layers other than the uppermost layer may be removed. For example, after the conductive GaN layer is formed as the uppermost layer, all of the layers from the base substrate to the GaN layer under the conductive GaN layer may be removed so that the conductive GaN layer is obtained as a substrate for use in a semiconductor element.
(vii) Preferably, a portion of the base substrate including the surface of the base substrate, on which the porous anodic alumina film is formed, is made of one of GaN, sapphire, SiC, ZnO, LiGaO2, LiAlO2, ZrB2, GaAs, GaP, Ge, and Si. For example, the entire base substrate may be made of one of GaN, sapphire, SiC, ZnO, LiGaO2, LiAlO2, ZrB2, GaAs, GaP, Ge, and Si. Alternatively, the base substrate may be constituted by a main portion made of one of GaN, sapphire, SiC, ZnO, LiGaO2, LiAlO2, ZrB2, GaAs, GaP, Ge, and Si, and a GaN layer formed on the main portion.
(3) According to the third aspect of the present invention, there is provided a semiconductor element comprising a substrate and semiconductor layers formed on the substrate. The substrate includes: a base substrate having a surface on which a plurality of pits are formed by etching using a porous anodic alumina film having a plurality of pores as a mask; and a GaN layer formed on the surface of the base substrate by crystal growth.
According to the fourth aspect of the present invention, there is provided a semiconductor element comprising a substrate and semiconductor layers formed on the substrate. The substrate is produced by forming a plurality of pits on a surface of a base substrate by etching using a porous anodic alumina film having a plurality of pores as a mask, forming a GaN layer on the surface of the base substrate by crystal growth, and removing the base substrate so as to leave the GaN layer as the substrate.
In the substrate according to each of the fourth aspect of the present invention, all of the layers from the base substrate to an arbitrary layer under the uppermost layer may be removed.
According to the fifth aspect of the present invention, there is provided a semiconductor element comprising a substrate and semiconductor layers formed on the substrate. The substrate is produced by forming at least one first GaN layer on the base substrate, forming on the at least one first GaN layer a second GaN layer which is doped with a conductive impurity, and removing the base substrate and the at least one first GaN layer so as to leave the second GaN layer as the substrate. Before each of the at least one first GaN layer is formed on a surface of one of the base substrate and the at least one first GaN layer located under each of the at least one first GaN layer, a plurality of pits are formed on the surface by etching using a porous anodic alumina film having a plurality of pores as a mask.
(4) According to the sixth aspect of the present invention, there is provided a substrate for use in a semiconductor element, comprising: a base substrate having a surface on which a plurality of pits are formed by etching using a porous anodic alumina film; and a GaN layer formed on the surface of the base substrate so that the plurality of pits are partially filled with the GaN layer and a plurality of spaces are left in the plurality of pits.
According to the seventh aspect of the present invention, there is provided a substrate for use in a semiconductor element, comprising: a base substrate having a surface on which a plurality of pits are formed by etching using a porous anodic alumina film; and a GaN layer formed on the surface of the base substrate so that the plurality of pits are fully filled with the GaN layer.
(5) The advantages of the present invention are as follows.
(i) In the process according to the first aspect of the present invention, a porous anodic alumina film having a great number of minute pores is formed on a surface of a base substrate, and the surface of the base substrate is etched by using the porous anodic alumina film as a mask so as to form a great number of pits on the surface of the base substrate. Then, the porous anodic alumina film is removed, and a GaN layer is formed on the surface of the base substrate by crystal growth. Therefore, the density of the nuclei for growth can be reduced compared with the conventional techniques, and thus it is possible to form a GaN layer including a wide low-defect region.
If, as in the cases of the conventional techniques, the GaN layer is formed by the selective lateral growth using the stripe areas as nuclei for growth, the spaces between the nuclei are bridged before the grown nuclei become large since the density of the nuclei for growth is high. Therefore, the defect density in the bridged regions becomes high. On the other hand, since, in the process according to the first aspect of the present invention, the surface of the base substrate is etched by using the porous anodic alumina film, a great number of minute pits can be formed on the surface of the base substrate, and the GaN layer is grown from the surface of the base substrate other than the minute pits. Thus, the density of the nuclei for growth can be reduced. In addition, since the unetched portions of the surface of the GaN layer other than the plurality of pits are connected to each other, lattice plane inclination which occurs when a crystal is largely grown from a nucleus for growth can be reduced, and defects in the bridged portions can also be reduced. Consequently, it is possible to form a GaN layer including a wide low-defect region. Further, even when the thickness of the GaN layer is increased, it is possible to obtain a GaN layer including a wide low-defect region.
Since a GaN layer including a wide low-defect region can be obtained, it is possible to obtain highly reliable semiconductor element by using a substrate produced by the process according to the first aspect of the present invention.
(ii) In the process according to the second aspect of the present invention, a porous anodic alumina film having a great number of minute pores is formed on a surface of a base substrate, and the surface of the base substrate is etched by using the porous anodic alumina film as a mask so that a great number of pits are formed on the surface of the base substrate and the porous anodic alumina film is etched off. Then, a GaN layer is formed on the surface of the base substrate by crystal growth. Therefore, it is possible to form a GaN layer including a wide low-defect region for a similar reason to the process according to the first aspect of the present invention.
In particular, in the process according to the second aspect of the present invention, the porous anodic alumina film which is used as a mask is removed at the same time as the formation of the plurality of minute pits on the surface of the base substrate. That is, the step for removing the porous anodic alumina film can be dispensed with, and the production process can be simplified.
(iii) Since a GaN layer including a wide low-defect region can be obtained by the process according to the first or second aspect of the present invention, it is possible to obtain highly reliable semiconductor element by using a substrate produced by the process according to the first or second aspect of the present invention.
(iv) When the diameters of the minute pores arranged on the surface of the base substrate are 10 to 400 nm, and the total area of the minute pores is 50 to 90% of the area of the surface of the base substrate, the density of the nuclei for growth can be more effectively reduced.
(v) When a conductive GaN layer which is doped with a conductive impurity is formed as an uppermost layer, a substrate having a further lower defect density can be produced for use in a semiconductor element. Therefore, for example, a dark current in a light receiving element can be reduced, and the performance of an electronic device or the like can be improved.
(vi) When the conductive GaN layer is formed as the uppermost layer, and thereafter all of the layers from the base substrate to the GaN layer under the conductive GaN layer is removed so that the conductive GaN layer is obtained as a substrate for use in a semiconductor element, and a semiconductor element such as a semiconductor laser device is produced by forming semiconductor layers including an active layer and the like on the substrate, an electrode can be formed on a back surface of the substrate, and therefore the process for producing the semiconductor element can be simplified.
(vii) The substrate according to the fifth aspect of the present invention comprises a base substrate having a surface on which a plurality of pits are formed by etching using a porous anodic alumina film, and a GaN layer formed on the surface of the base substrate so that the plurality of pits are partially filled with the GaN layer. Since the GaN layer formed on the surface of the base substrate on which the plurality of pits are formed by etching using the porous anodic alumina film has a wide low-defect region, a highly reliable semiconductor element can be produced by using the substrate according to the fifth aspect of the present invention.
In addition, since a plurality of spaces are left in the plurality of pits, it is possible to relax distortion or the like which is caused by the difference in thermal expansion between the substrate and the GaN layer when the temperature rises or falls, and suppress the occurrence of defects which are produced by the distortion.
(viii) The substrate according to the sixth aspect of the present invention comprises a base substrate having a surface on which a plurality of pits are formed by etching using a porous anodic alumina film, and a GaN layer formed on the surface of the base substrate so that the plurality of pits are fully filled with the GaN layer. Since the GaN layer formed on the surface of the base substrate on which the plurality of pits are formed by etching using the porous anodic alumina film has a wide low-defect region, a highly reliable semiconductor element can be produced by using the substrate according to the sixth aspect of the present invention.